1. Field of the invention
The present invention relates to a recording apparatus having discoid record mediums and more specifically to a recording apparatus executing on-track control of heads based on servo information recorded on the record mediums.
2. Description of the Related Art
In recording apparatuses such as a Hard Disk Drive (hereinafter, referred to as “HDD”) having discoid record mediums, the position of a head is controlled according to the servo information written in storage mediums such as disks. A head is a device for detecting magnetic signals recorded on disks and for recording magnetic signals on disks. Write-in of information and read-out of information recorded on a disk is enabled by sliding the head on the disk. The servo information is recorded in a servo pattern written radially on the surface of a disk.
FIG. 1 illustrates the state of the surface of a disk inside an HDD. A disk 2 is a discoid record medium rotated by a spindle motor 1 having a cylindrical shape. The central shaft of the spindle motor 1 is herein referred to as “rotation center”.
A servo pattern 5 including servo information is written radially on the surface of the disk 2. FIG. 1 is depicted assuming that the center of the servo pattern 5 and the rotation center are at the same position.
Tracks 3 in the form of concentric circular tracks are defined in the disk 2 such that points having the same distance from the center of the servo pattern 5 written radially have a same track number. Each track 3 has a predetermined width. Each track 3 is further divided into a plurality of sectors 4 and each sector 4 is managed with a sector number.
Since reading and writing of data are executed sector-by-sector, a head 6 is controlled such that it follows on a same track, and detects signals recorded in sectors 4. The servo information contained in the servo pattern 5 includes a track number and a sector number which indicate the part where the radially written servo pattern 5 intersects tracks, and a signal used for determining whether the head 6 are shifted from a track center 8 or not.
In order to control the head 6 such that it follows a same track, when the head passes the servo pattern 5, the servo information is read out. Whether or not the track number contained in the servo information is the track to follow and whether or not the head is shifted from the center 8 of the track are determined. When the head is not shifted from the center of the track to follow, the state is referred to as “the head is on-track” and, when it is shifted from the center of the track to follow, the state is referred to as “the head is off-track”.
The head 6 is supported by a head arm 7 and the head arm 7 is connected to a voice coil motor 9. The head 6 can move over a predetermined area on the surface of the disk 2 and read-in and write-out of data on the disk 2 can be executed by virtue of driving the head arm by the voice coil motor 9 and rotations of the disk 2.
In order to write the above-described servo information onto the disk 2, there are two (2) methods: one is a method in which, after the disk 2 has been installed into the HDD, a program for writing the servo information is started up and the servo pattern 5 is written on the surface of the disk with the head 6; and the other is a method in which, after the servo pattern 5 has been written on the surface of the disk with a dedicated device for writing the servo information (hereinafter, referred to as “STW (=Servo Track Writer)”), the disk 2 is installed into the HDD.
In either method, due to the vibration of the disk 2 or the vibration of the head 6 generated when the disk 2 is rotated during the write-in of the servo information, the tracks 3 defined by the servo information actually written may not be circles which center is the rotation center of the disk 2 and the tracks may windle. In the latter method, due to the installation error generated when the disk 2 is installed into the HDD after the servo pattern 5 has been written by the STW, the center of the servo pattern 5 recorded radially and the rotation center of the disk 2 do not coincide with each other and the centers are shifted from each other. Therefore, each track 3 defined by the servo information actually written may also not be a circle having its center at the rotation center of the disk 2.
The heads 6 are disposed such that they sandwich the disk 2 that is rotating. Since the number of the heads capable of being controlled at one time is one (1), the head to be controlled may be switched from the head on the face to the head on the back in order to read and write data on the back surface of the disk 2. In the case where a plurality of disks 2 are present in the HDD, the head to be controlled may be switched to one of the heads disposed such that they sandwich another disk 2.
In the case where each track 3 is not a circle having its center at the rotation center of the disk 2, the head position stabilizing time at the time when the head 6 has been switched becomes considerably long. This is because the track number does not differ between before and after the switching of the head if each track 3 is a circle having its center at the rotation center of the disk 2, however, the track number differs between before and after the switching of the head if each track 3 is not a circle having its center at the rotation center of the disk 2 and, therefore, the head is controlled, after the switching, to return to the track having the same track number as the track number before the switching.
In order to solve this task, a method has been proposed for causing the head 6 to follow virtual circles around the rotation center of the disk 2, not that the head 6 is controlled according to the actually written servo information (see Patent Document 1). In Patent Document 1, virtual concentric circular orbits around the rotation center of the disk 2 are set and the virtual circular orbits are used as the virtual tracks. Data is read and written along the virtual tracks using the correspondence data to correspond the tracks defined by the servo information to the virtual tracks.
However, the place to store this correspondence data is the problem. In the conventional technique, the correspondence data is recorded in each sector, otherwise, a conversion table clarifying the correspondence data for each sector is recorded only in a non-volatile memory such as a PROM (Programmable Read Only Memory).
The tracks defined by the servo information cross the virtual tracks and they interfere with each other. Therefore, the above-described correspondence data is overwritten by data written along the virtual tracks. As a result, the correspondence data can not be read out and the control along the virtual tracks may not be able to be executed. Furthermore, in the case where the above-described conversion table is stored only in the non-volatile memory such as a PROM, a failure of the non-volatile memory having occurred after shipment can not be coped with and, in this case, the control along the virtual track also can not be executed.
In the conventional technique, in any of the above cases, the correspondence data or the conversion table is not stored at the place where it can be securely read out and, when any of the above data can not be read out, a task that the head control along the virtual track can not be executed has arisen.
(Patent Document 1) Japanese Patent Application Laid-Open (Kokai) Pub. No. H9-330571